The present invention relates to torque transmitting fluid couplings, and more particularly, to such couplings which utilize internal valving, whereby the fluid coupling may be in either an engaged or disengaged condition, depending upon the position of the valving.
Fluid couplings of the type to which the present invention relates are well known in the art and may be better understood by reference to U.S. Pat. Nos. 3,055,473; 3,174,600; and 3,339,689, all of which are assigned to the assignee of the present invention. Briefly, such fluid couplings typically include an output coupling member and a cover which cooperate to define a fluid chamber, a valve plate dividing the fluid chamber into an operating chamber and a reservoir chamber, and an input coupling member disposed within the operating chamber and rotatable relative to the output coupling member. The input and output coupling members define a shear space such that rotation of the input member causes viscous fluid in the shear space to exert a viscous drag on the output member, causing it to rotate. The valve plate defines a fill orifice, and a valving arrangement controls the flow of fluid from the reservoir chamber, through the fill orifice, into the operating chamber. Typically, the valving is temperature-responsive, as is illustrated in the above-cited patents, such that below a certain ambient temperature, the valving is closed, most of the viscous fluid is discharged from the operating chamber to the reservoir chamber and the fluid coupling is considered to be "disengaged". Above the predetermined temperature, the valving gradually opens and viscous fluid is permitted to flow from the reservoir into the operating chamber, filling the shear space, such that the coupling is "engaged".
Conventional fluid couplings of the type to which the present invention relates have been provided with relatively small clearances between the outer periphery of the input member and the inner periphery of the output member, partly because the viscous fluid between these adjacent peripheries acts as a fluid bearing, and partly to maximize the available shear surface and the torque transmitting capacity. Therefore, although the present invention may be utilized in fluid coupling devices of many different embodiments, it is especially useful in those in which the outer periphery of the input member and the inner periphery of the output member are closely spaced apart. It is also especially useful in those in which some form of valving is provided to control the flow of fluid into the operating chamber, such that the coupling may be utilized in either an engaged or a disengaged condition.
Conventional fluid couplings have been of the type referred to as "full OD", i.e., the outer periphery of the input member is cylindrical and has a maximum diameter over the entire axial extent of the outer periphery. As noted previously, a full OD input member provides maximum torque transmission when the fluid coupling is engaged. With the coupling disengaged, however, several problems arise in connection with the use of the full OD input member. One of these is the "cold-start" condition which arises after the coupling has been inoperative for a period of time and fluid has leaked from the reservoir into the operating chamber, causing the coupling to operate as though it were engaged when it is supposed to be disengaged. Upon start-up of the coupling under this condition, it typically takes several minutes for enough of the fluid to be discharged from the operating chamber back into the reservoir chamber to reduce the speed of the output member to its normal, disengaged level. During this period of time, operation of the coupling is normally not desired, e.g., the coupling is driving the radiator cooling fan of a vehicle engine and no cooling is required upon initial start-up of the vehicle engine. Moreover, the continued, engaged operation of the coupling for a period of several minutes, typically at speeds well above 1,000 rpm, results in an objectionable noise level, especially when the engine is warming up at fast idle.
A related problem is the output speed level of the coupling in the disengaged condition. A relatively higher disengaged output speed results in a relatively higher horsepower consumption by the coupling and the associated cooling fan with no resultant benefit. Finally, the problem of acceleration overshoot is common in fluid couplings of this type. Acceleration overshoot occurs as the input speed rises from a low level and the output speed temporarily rises well above the normal disengaged speed level before dropping down again to the normal level.
In an attempt to overcome the problems associated with a full OD input member, those working in the art have developed and commercialized a "stepped OD" input member, a general example of which is illustrated in U.S. Pat. No. 3,613,847. The assignee of the present invention has commercialized a coupling having a stepped OD input member, but with the step adjacent the forward surface of the input member, rather than adjacent the rearward surface as in the cited patent. Although the stepped OD input member overcomes certain disadvantages of the full OD configuration, its use introduces certain additional problems. The step (typically, 0.050 inches wide .times. 0.050 inches high) is susceptible to damage if subjected to normal handling during manufacture, or conversely, requires special handling during manufacture to avoid damage to the step. In addition, if it is desired to further improve the disengaged operating characteristics (i.e., reduce cold-start time, reduce disengaged speed level, and reduce acceleration overshoot), it is necessary to increase the height of the step (i.e., reduce the diameter of the input member adjacent the step). However, increasing the height of the step further weakens the outer periphery of the input member and changes its ability to withstand handling, or changes the handling procedures required to avoid damage.